Oscar F. Dame
The ordinary induction cell under certain adjustments gives a flaming spark, which we might call high frequency in comparison with the normal output, when we adjust the vibrator at its highest speed and keep the battery amperage unchangeable. Such a spark does not pass in a straight line from one terminal to the other, but assumes a caterpillar shape, carrying with it much more heat than the ordinary discharge, particularly when we close up the gap to one-quarter of its maximum distance.
Should we bridge a small Leyden jar across the secondary terminals, we would note a decided change in the character of the spark, it now being a blue-white instead of a flaming yellow, and decidedly noisy. Such a spark has greatly increased in frequency, and is alternating in character. The experiments are of great interest, in that they permit of the construction of apparatus similar to that built and operated by Tesla in his most original experiments some years ago. His experiments have been very numerous, and to possess
all the apparatus necessary for them would involve a considerable outlay, but it is possible for any one possessing an induction coil giving over 1-in. spark, to produce these alternating currents of high-frequency at very small expense, using such materials as may be found in nearly every home workshop.
The first requisites are two glass condensers or capacities. These may be in the form of Leyden jars or glass plate condensers. Leyden jars are easy to make but not always neat in appearance when home-made, as tinfoil cannot be applied evenly to glass cylinders without considerable trouble. The writer found it possible to construct condensers out of large panes of sheet glass, extra thin, of a size about 18 by 24 in. Each side is coated with tinfoil to within 2 in. of the
edges on both sides. If it is not possible to secure large sheets of tinfoil, smaller pieces overlapping one-another to make perfect contact may be used. The pieces of glass are carefully shellacked on both sides, particularly at the edges, but with the exception of a central spot on each side exactly 2 in. in diameter. The glass is then mounted on a wooden baseboard, between two supports made of dry wood or rubber, and with adjustable plates, which also serve as contact plates and terminals. There is no particular method of designing this support, but a simple way is to employ two long machine screws for adjusters. These may be tightened with a screw-driver, as required. A side view of one of these condensers is shown in Fig. 1. In connecting with the induction coil it is necessary to use $ in. balls on the spark gap and separate same between 1/4 and 3/8 in. Placing the two plate condensers upon the table, two feet apart, the outer terminals are connected by metal chains to the spark gap knobs above referred to. To the inside terminals, thereby completing a circuit through the condensers, will be connected a "resonator" which is probably a new device to many Amateur Work readers, but which will be found available in all high frequency and wireless telegraph experiments. A resonator is a solenoid or spiral of heavy wire of low resistance, the turns of which are fully 1/4 of an inch apart. The resonator here required should be of No. 12 bare copper wire, wound in 40 turns about a cylinder of dry wood 6 in. in diameter. As the wire is to be wound in open spiral it will be advisable to have the cylinder at least 18 in. high, and a fair estimate of wire length is 70 feet. This wire is tacked to the wood at the two ends, and the entire resonator placed in a pan and basted with boiling paraffine until the wood is well filled with the wax. Any wax that may stick to the outside of the wire readily falls off.
The writer used birch wood in constructing his resonator, because it is very porous, taking up plenty of wax and so permitting a more thorough insulation. This cylinder is mounted on a baseboard for convenience in handling. The two connecting wires are in reality light brass chains with suspender clips fastened to the ends. In this way, any part of the wire may be used for the resonance. In fact, it requires some experiment to ascertain just how much of the spiral is to be included in the condenser circuit to give the proper frequency. This resonator when used with a coil ordinarily giving from one to three inches spark, will deliver a volume of radience of surprising intensity, permitting the performance of the most curious and instructive experiments. In the dark we may observe all around it a very intense electro-static field, and it is possible by simply holding in the hand such objects as incandescent lamp bulbs and Geissler tubes to render same luminous. The larger the coil in use he better the results. The effects shown in the illustration were from a 4-in. coil, but similar and just as brilliant results of a lesser magnitude were obtained from a 1 1/2 in. coil working on six dry cells. A diagram of all connections is shown in Fig. 3.